Recently, the pressure-sensitive adhesive sheets have been employed in various aspects such as fixation (bonding), conveyance, protection, decoration, and the like of articles. Typical examples of the pressure-sensitive adhesive sheets include those provided with a pressure-sensitive adhesive layer formed by using an acrylic pressure-sensitive adhesive composition. This acrylic pressure-sensitive adhesive composition is configured such that it may form a pressure-sensitive adhesive layer having an acrylic copolymer as a base polymer. This acrylic copolymer generally contains alkyl (meth)acrylate as a major ingredient (a main component), and further contains the copolymerization composition containing carboxyl group-containing monomers such as acrylic acid, and the like, for the purpose of improving the pressure-sensitive adhesive performances, and the like.
However, when a pressure-sensitive adhesive sheet used for the articles having a metal surface (electronic parts, and the like) has a copolymerization composition containing a carboxyl group-containing monomer as described above, the carboxyl group in the base polymer may be responsible for the corrosion of the metal surface. Accordingly, the pressure-sensitive adhesive composition for forming a pressure-sensitive adhesive layer provided in a pressure-sensitive adhesive sheet for this application (particularly, application for directly attaching to a metal surface) is preferably in a constitution enabling the formation of a pressure-sensitive adhesive layer containing, as a base polymer, an acrylic copolymer having a copolymerization composition containing no carboxyl group-containing monomer. As the background art documents regarding these kinds of technologies, JP-A-2007-63536, JP-A-2005-325250 and JP-A-2000-303045 may be exemplified.
However, a pressure-sensitive adhesive composition containing no carboxyl group-containing monomer has a tendency of giving insufficient adhesion characteristics for a pressure-sensitive adhesive sheet formed from the composition. Regarding this, JP-A-2007-63536 describes copolymerization with a specific maleimide-based compound, JP-A-2005-325250 describes copolymerization with nitrogen atom-containing monomers such as N-acryloylmorpholine, and JP-A-2000-303045 describes use of specific monomers such as phenoxyethyl acrylate as main monomer components. However, even in the pressure-sensitive adhesive sheets formed using these technologies, there is room for improvement in adhesive performance. For example, it would be useful to provide a pressure-sensitive adhesive composition that exhibits a desired cohesive force without under the use of a carboxyl group-containing monomer, and further is capable of forming a pressure-sensitive adhesive sheet having increased repelling resistance (adhesiveness on a curved surface).